Divers start each dive from a certain starting point, such as a boat or an equivalent base station, from which the diver often moves further away during diving. Thus, at the end of the diving, the diver must find his way back to this starting point, and therefore the diver must be capable of orientating or, more appropriately, navigating under water. Most commonly, this navigation has been carried out by means of a compass, and in some cases, in particular in conditions with poor visibility, a rope has also been used as an aid, which rope has been attached both to the starting point, such as a boat, and to the diver.
When the movement takes place from the base station substantially only in one direction, by means of a compass it is rather easy to find the path back to this base station. The use of a compass in navigation so as to find the base station, i.e., the starting point, however, becomes considerably more difficult if the route passed by the diver has not been linear, if there have been sea currents in the area, or if the base station, such as a boat, has moved away from its place during diving. Moreover, the use of a compass is quite difficult for most divers, for a number of reasons, and its use requires constant attention. Further, when a compass is used, it is necessary to remember any turns that have been made during the diving, to take into account the effects of sea currents, etc.